Power looms have a so-called shed forming device, which serves to guide warp yarns outward and upward or downward from the warp yarn plane in accordance with a binding pattern, so that the spread-apart warp yarns create what is known as a shed. A weft yarn is introduced into this shed (weft insertion). Shedding can be done for instance by means of so-called heddles, with one warp yarn passing through the eye of each heddle. The heddles are held on a frame that is called a heddle shaft. The heddle shaft must be moved up and down in rapid succession in accordance with the pace of operation of the power loom. As a rule, modern power looms have a plurality of heddle shafts, which are located close together and in line with one another. The various shafts are assigned to different warp yarns. Depending on which shafts are guided upward or downward, different sheds can be formed in order to generate the desired weave structure (weave pattern) between the warp yarns and weft yarns. For driving the shafts, as a rule so-called shaft machines are used, which derive the shaft motion from the main drive of the power loom by suitable gear means.
Moving the shafts individually by means of suitable servo drives has already been proposed multiple times. For instance, German Patent Disclosure DE 198 21 094 A1 discloses an electromagnetic drive for the purpose. In one embodiment, the shaft is connected to two electromagnetic drive mechanisms, which act as linear drives. Tension and presser bars connect the movable member of the electromagnetic drive to the shaft. The electromagnetic linear drives are located below the shaft.
From the same reference, it is known to locate the electromagnetic linear drive next to the shaft, and to convert the horizontal working motion of the drive into the vertical working motion of the shaft via a suitable gear.
This principle meets limits if a plurality of shafts must be located close together and correspondingly driven. Typically, the shafts are at a spacing of 12 mm. The space available for the electromagnetic linear drives is thus extremely restricted, so that practical solutions to the problem are hardly achievable.
From Japanese Patent Disclosure JP 2003-89940 A, it is known to drive heddle shafts of a power loom by means of servo motors. To that end, one servo motor is assigned to each heddle shaft. The servo motors are located in two planes one above the other in a drive chest located correspondingly laterally next to the power loom. Each servo motor actuates one connecting rod via a cam. Angle levers located below the heddle shafts deflect the approximately horizontally oscillating connecting rod motion into a vertical oscillation. The two angle levers are joined together by a tension and compression rod.
A similar arrangement is known from European Patent Disclosure EP 1 215 317 A2. However, this reference also discloses the disposition of servo motors in more than two levels one above the other; the servo motors may be offset from one another or vertically flush with one another.
In the drive mechanisms of the last two references named, difficulties and limitations can arise in terms of the operating speed, because of the inherently inertial masses of the gear elements and from bearing plays.
European Patent Disclosure EP 0 879 909 A1 proposes for this purpose that the heddle shafts be moved by means of linear direct drives or electric linear motors. Because of the close spacing of the heddle shafts, or the slight distance between them, this runs up against the same problems in terms of accommodating the linear drives.
German Patent Disclosure DE 101 11 017 A1 seeks to overcome these disadvantages with special electric motors constructed in disklike fashion, whose rotor forms a long lever. The lever is connected to the heddle shaft via a connecting rod. The special motors can be placed in pairs, diametrically opposite one another, and can also be placed in a plurality of planes. As a result, twice or four times the axial heddle shaft thickness is available for the axial structural length of the motors. Nevertheless, in this concept, the axial length of the motors is limited. Moreover, at least if the motors are located in different planes, different connecting rods are different lengths, which again can cause problems.
Against this background, it is the object of the invention to create a shed forming device which is improved in terms of the embodiment of its drive mechanism.